Origin: http://ubuntuforums.org/showthread.php?t=1181014

Lock X mouse pointer to a single area or screen.

### NOTE

The ScreenIdx refers to the ID of the screen. The ID of primary screen
is always `0`. The other screens increases the ID from left to right.

```
|---------|     |---------|      |---------|      |---------|
|    1    |     |    2    |      | Primary |      |    3    |
|         |     |         |      |    0    |      |         |
|---------|     |---------|      |---------|      |---------|
```
